This invention relates to solenoids operated by alternating electric current in which a ferromagnetic plunger is drawn into a bore by current flowing through a coil surrounding the bore. Clearance between the plunger and bore is provided to permit free movement of the plunger and absorb manufacturing tolerances. The inner axial end of the plunger usually contacts a ferromagnetic backstop in the bore when the plunger is fully drawn into the bore, the backstop being connected to a ferromagnetic frame which supports the bobbin and acts as a low reluctance path for the magnetic lines of force.
Shading coils have been used to provide a continuity of magnetic attraction as the alternating current in the coil passes through the zero point of its cycle by delaying the flux in the shaded portion of either the plunger or backstop where the shading ring is mounted.
Present designs of solenoids are vulnerable to buzzing when the alignment of the backstop and core face is not perfect. Elaborate machining and sizing operations are used in an attempt to provide the required alignment to prevent buzzing. These attempts have added to cost and have not eliminated the problem of buzzing caused by manufacturing tolerances or wear in use.
In one unsuccessful attempt to eliminate buzzing, the adjacent axial end surfaces of the plunger and backstop have been shaped as matching spherical surfaces to permit contact between the shading coil and the backstop even when the plunger is slightly tilted with respect to the bore. This arrangment is disclosed in U.S. Pat. No. 3,185,902 to R. G. Fecho et al. However, the use of spherical surfaces on the axially adjacent ends of the plunger and backstop also fails to eliminate buzzing.